It is conventional that the metallic silicon or ferrosilicon is provided by reduction of silica (silicon dioxide) in the aid of coke and/or coal utilizing arc furnace. In the reduction process, not only the metallic silicon but also impurity components, such as iron, titanium, aluminum and so forth, are reduced. By the presence of such impurity components to be reduced with the metallic silicon, the purity of the metallic silicon to be produced has been limited at about 98% to 99%.
In the recent years, there has been an increasing demand for metallic silicon of substantially high-purity, e.g. higher than or equal to 99.999%. Such high-purity metallic silicon has numerous application, such as for solar cells. In order to respond to such a demand, a process has been developed and proposed. Such a proposed process utilizes refined natural silica as the source of silicon dioxide. This refined natural silica is generally powdered or granular with a grain size of less than several millimeters. Therefore, in order to use this refined silica, an extra process step was needed to sufficiently increase the grain size of the silica since such small grain size silica could not be used for such reducing process otherwise.
For example, the Japanese Patent First (unexamined) Publication (Tokkai) Showa 57-111223 proposes a method for reducing the refined natural silica. The process proposed in the Tokkai Showa 57-111223 employs a process step for preparing material silica block of 3-12 mm in diameter. This clearly increase ineconomically the process steps in the production of the high-purity silicon. In addition, during the preparation of the material silica blocks, an impurity tends to be contained in the block for causing lowering of the purity of the material silica.
Another improvement has been proposed in the Japanese Patent First Publication (Tokkai) Showa 58-69713. In the disclosed process, the reaction between silica and carbon takes place in a high-temperature plasma jet which transports the resultant product onto a carbon layer. In this proposed process, a large amount of silicon carbide is created as a result of reaction with the carbon layer. The created silicon carbide tends to accumulate within the carbon layer and fill interstices between the carbon grains, which inhibits further reaction.
A further proposal has been disclosed in the European Patent First (unexamined) Publication No. 02 08 567. The disclosed method and apparatus perform a production of high-purity metallic silicon by subjecting a stream of oxides of silicon, (e.g. in an aerosol) to reaction heat in the presence of a mixture of oxides of silicon and a material of the group including carbon and metallic carbide. The silicon monoxide produced by the reaction is scavenged from exhaust gas leaving the reaction chamber, re-condensed, and returned to reaction chamber. Such prior proposed method requires scavenging of the silicon monoxide containing exhaust gas to be recirculated into the reaction chamber for obtaining satisfactorily high yield.
The process disclosed in the European Patent First Publication No. 02 08 567 is established based on that the metallic silicon is produced generally through the chemical reaction caused during the production process: EQU SiO.sub.2+ 2C.fwdarw.Si+2CO . . . (1)
In practice, combination of reactions will occur during the metallic silicon production process, which reactions can be illustrated by: EQU SiO.sub.2+C.fwdarw.SiO+CO . . . ( 2) EQU SiO.sub.2+ 3C.fwdarw.SiC+2CO . . . (3) EQU 2SiO.sub.2 +SiC.fwdarw.3SiO+CO . . . (4) EQU SiO.sub.2 +Si.fwdarw.2SiO . . . (5) EQU SiO+2C.fwdarw.SiC+CO . . . (6) EQU SiO+C.fwdarw.Si+CO . . . (7) EQU SiO+SiC.fwdarw.2 Si+CO . . . (8)
When a powder state silica is introduced into an electric furnace, in, which the aforementioned combination of reactions is occurring, large volume of silicon monoxide SiO is produced through the reaction of the formula (2) since the powder state silica has better reaction ability than that of block form silica. Part of the silicon monoxide tends to be blown away from the furnace with the exhaust gas.
Therefore, the foregoing European Patent First Publication No. 02 08 567 discloses a process including injecting the silica through the bottom of the furnace to induce reactions of formulae (2) and (4) at around arcing region to generate silicon monoxide. The silicon monoxide thus generated reacts with carbon or silicon carbide charged through the top of furnace at a reaction temperature about 1800.degree. C. or higher to induce reactions of the formulae (6) and (8) to generate the metallic silicon and silicon carbide. As will be appreciated, the reactions of formulae (2) and (4) are reactions for producing silicon monoxide and carbon monoxide and the reactions of formulae (6) and (8) are reactions for producing carbon monoxide with consumption of the silicon monoxide. In thermodynamical analysis, it was found that the ratio of silicon monoxide versus carbon monoxide in the reactions of formulae (2) and (4) was in a range of 1.67 to 2.48 in the equilibrium state whereas the ratio in the reaction of formulaa of (6) and (8) was in a range of 0.45 to 0.95. Therefore, in the process including recirculation of silicon monoxide, only about 20 to 50% of silicon monoxide can be used for the production of the metallic silicon. It is also recognized that the reduction of the ratio of silicon monoxide versus carbon monoxide in the reactions of the formulae (6) and (8) is caused by dilution of gaseous state silicon monoxide with carbon monoxide gas as by-product.
Therefore, as will be appreciated from the foregoing discussion, the process proposed in the European Patent First Publication No. 02 08 567 is not yet satisfactory in viewpoint of production yield.